Turbocharging machinery is well-known and commonly used in the internal combustion engine industry to pressurize intake air entering the engine combustion chambers and thereby increase the efficiency and power output of the engine. In general, pressurizing the intake air increases the quantity of air entering the engine cylinders during the intake stroke, and this allows more fuel to be utilized in establishing a desired air/fuel ratio. Increased available engine output torque and power thereby results.
Conventional turbochargers for internal combustion engines include a turbine disposed in the path of exhaust gas exiting the engine exhaust manifold, wherein the turbine typically includes a wheel that is rotated via the flow of exhaust gas thereby. The turbine wheel is rotatably coupled to a wheel of a compressor disposed in-line with the air intake system of the engine. Rotation of the turbine by the exhaust gas flow causes the compressor wheel to likewise rotate, wherein rotation of the compressor wheel acts to increase the flow of fresh air to, and consequently the air pressure within, the air intake system. Generally, the rotational speed of the turbocharger turbine and compressor wheels, and hence the air pressure within the air intake system, is proportional to the flow rate of exhaust gas, which is itself proportional to engine speed.
In the operation of turbochargers of the type just described, a condition known as turbocharger compressor surge is known to occur under certain engine and air handling system operation. Generally, turbocharger compressor surge occurs when the accumulated pressure in the intake manifold exceeds the ability of the compressor to sustain positive air movement. When this occurs, intake manifold air pressure decreases by an amount generally proportional to the intensity of the surge condition.
A number of engine and air handling system conditions contribute to, and define, turbocharger compressor surge including, for example, engine speed, engine fueling rate, turbocharger speed, mass flow rate of intake air, intake manifold pressure, intake manifold volume, intake manifold temperature, and the like. In engines including exhaust gas recirculation systems, another engine operating parameter that impacts and defines turbocharger compressor surge is the flow rate of exhaust gas recirculated to the intake manifold, which affects the mass flow rate of intake air and intake manifold pressure. What is therefore needed is a system for monitoring EGR flow rate, or alternatively the EGR fraction, which is the fraction of recirculated exhaust gas relative to the total air charge entering the intake manifold, and to control such EGR flow rate or EGR fraction in a manner that avoids turbocharger compressor surge.